This invention relates to a thermal transfer sheet, and more particularly to a thermal transfer sheet capable of forming a recording image having excellent color density, clearness, and fastnesses, particularly light fastness.
Heretofore, various thermal transfer processes have been known. Of these, there has been widely used a sublimation transfer process wherein a sublimable dye is used as a recording agent; it is carried on a base sheet such as paper to form a thermal transfer sheet; this thermal transfer sheet is superposed on a transferable material which can be dyed by the sublimable dye, for example, a polyester woven fabric or the like; and a heat energy is applied in the form of a pattern from the back surface of the thermal transfer sheet to transfer the sublimable dye to the transferable material.
Recently, there has been a process for forming various full color images on materials such as paper and plastic films using the thermal transfer process of sublimation type described above. In this case, a thermal head of a printer is used as heating means, multi-color dots such as three-color or four-color dots are transferred to the transferable material by heating for an extremely short period of time, and the full color images of a original are reproduced by the multi-color dots.
The images thus formed are very clear since the colorant used is a dye. Because the transparency is excellent, the images obtained have excellent neutral tint reproducibility and gradation, they are similar to the images obtained by the prior offset printing and gravure printing and high performance images comparable to full color photographic images can be formed.
However, the most important problems of the thermal transfer process described above are inferior color density and light fastness of the formed images.
That is, in the case of high-speed recording, it is required that the impartation of the heat energy be an extremely short period of time of subsecond. Accordingly, the sublimable dye and the transferable material are not sufficiently heated due to such a short period of time and therefore images having a sufficient density cannot be formed.
Accordingly, sublimable dyes having an excellent sublimation property have been developed in order to cope with such a high-speed recording, process. However, the dyes having an excellent sublimation property have generally a small molecular weight and therefore their light fastness is lack in the transferable material after transfer. Thus, the formed images are liable to be faded.
If sublimable dyes having a relatively high molecular weight are used in order to avoid such problems, images having a satisfactory density as described above cannot be obtained since the sublimation rate is inferior in the high-speed recording process as described above.